We examined the role of DNA methyltransferase 1 (DNMT1) in N-methyl- N′-nitro- N-nitrosoguanidine (MNNG)–induced malignant transformation of Kazakh esophageal epithelial (EE) cells to better understand the pathogenesis of esophageal cancer (EC). The 3-(4,5)-dimethylthiahiazo (-z-y1)-3,5-di- phenytetrazoliumromide method and colony formation assays were performed to determine the MNNG dose for malignant transformation. Colony formation assays showed the effects of different frequencies of MNNG exposure and different cell passages on malignant transformation. A nude mouse tumor experiment indicated the malignancy of Kazakh EE cells expressing high DNMT1 levels and of transformed cells. The result shows that when the dose, frequency, and time of MNNG exposure increased, cell morphology became irregular, cell-contact suppression disappeared, and cell tolerance and growth rate increased. Colony formation occurred in the Kazakh-DNMT1 group after 14 transfections and 27 passages. Significant differences in DNMT1 mRNA and protein levels were observed in different types of cells and tumor tissues ( F = 140.644, p < 0.001; F = 105.545, p < 0.001). Our study demonstrated that DNMT1 could promote MNNG to induce malignant transformation of EE cells, and this study will help understand EC better in order to develop appropriate treatment strategies.

The protective effects of folic acid on DNA damage and DNA methylation induced by N-methyl- N'-nitro- N-nitrosoguanidine (MNNG) in Kazakh esophageal epithelial cells were investigated using a 3 × 3 factorial design trial. The cells were cultured in vitro and exposed to media containing different concentrations of folic acid and MNNG, after which growth indices were detected. DNA damage levels were measured using comet assays, and genome-wide DNA methylation levels (MLs) were measured using high-performance liquid chromatography. The DNA methylation of methylenetetrahydrofolate reductase (MTHFR) and folate receptor- α (FR α) genes was detected by bisulfite sequencing polymerase chain reaction (PCR). The results showed significant increases in tail DNA concentration, tail length, and Olive tail moment ( p < 0.01); a significant reduction of genome-wide DNA MLs ( p < 0.01); and an increase in the methylation frequencies of MTHFR and FR α genes. In particular, significant differences were observed in the promoter regions of both genes ( p < 0.01). Our study indicated that a reduction in folic acid concentration promotes DNA damage and DNA methylation in Kazakh esophageal epithelial cells upon MNNG exposure. Thus, sufficient folic acid levels could play a protective role against the damage induced by this compound.